FIELD OF THE INVENTION
This invention relates to an optical disk apparatus wherein an optical disk is irradiated with a beam of light to make tracking control on the basis of the reflected light from the optical disk.
In an optical disk appartus which radiates laser light to the surface of a rotary recording medium to optically record or reproduce data, an automatic focus servo system is necessary which moves an objective lens of an optical pickup in accordance with vertical motion occurring in the rotating optical disk so that the data recording surface is always within the depth of focus of a laser spot. The automatic focus servo system consists of a voice coil type servo motor, for example, for moving up and down the objective lens, a focus error detecting portion and a servo amplifier for operating the servo motor in accordance with the detected focus error. Among these constituents, the focus error detecting portion is of particular importance. When adapted to the optical disk apparatus, it has desirably such a construction that the change of the reflected rays of light resulting from data pits formed on the data recording surface or from a pregroup forming tracks does not adversely affect a focus error signal. Moreover, it preferably has such a construction that even when any deviation occurs in the optical axis of the reflected rays of light from the data recording surface due to position errors of an optical system, for example, the deviation does not adversely affect the focus error signal.
Various system constructions have so far been proposed for a detector which detects the focus error described above. One of the systems converges the reflected rays of light from the data recording surface (the reflecting surface) by means of a lens and disposes a knife edge at the point of convergence so that only part of the reflected rays of light reaches a photodetector disposed at the rear of the knife edge. (Refer, for example, to "Nikkei Electronics", Nov. 21, 1983, pp. 201-202). This system can obtain a semicircular optical image that rotates in accordance with the focus deviation quantity on the photodetector, when a cylindrical lens is interposed between the convergent lens and the knife edge, and a high accuracy focus error signal can be obtained from a differential output of two split photodetectors. In accordance with this sytem, however, there occurs a problem that a change in the position of the knife edge due to thermal expansion or the like and a deviation of the optical axis of the reflected rays of light affect adversely the focus error signal because the focus error signal depends upon the relation of relative positions between the knife edge and the convergent light.
Another system is disclosed in Japanese Patent Laid-Open No. 77637/1984, for example. This system disposes an optical element on an optical path for converging the reflected rays of light in order to separate the center portion of luminus flux from its peripheral portion in different directions, receives these separated luminus fluxes by separate photodetectors and obtains the focus error signal from the difference between the outputs of the photodetectors. However, this system involves a problem that the error signal contains a noise component because the modes of appearance of changes in the optical quantity are different between the center portion and the peripheral portion of the reflected rays of light due to the data pits and the track described already.
Moreover, the conventional systems described above do not consider the offset of the tracking error signal due to the position error of the optical system.